1. Field
The present invention relates to systems and methods for protecting against damage caused by undesired side loads on rotatable equipment, such as may be applied to the booms of mobile cranes and digger derricks. In particular, the present invention relates to a system and method for allowing a boom to slip in response to excessive side loading, wherein the boom slips at varying side load torque thresholds according to a position of the boom.
2. Related Art
Mobile cranes, digger derricks and other types of rotating equipment are commonly used to perform many different jobs, including lifting loads, digging holes for utility poles and installing screw anchors for supporting the poles. Mobile cranes and digger derricks are typically mounted to mobile utility vehicles and include a rotating turret from which a boom extends. The boom may be raised, lowered, extended, and retracted to position its outboard end in various locations to perform hoisting or digging operations. Digger and auger assemblies may be coupled with the outer end of the boom for digging holes for utility poles and for installing screw anchors.
Mobile cranes, digger derricks and other rotating equipment are often subjected to excessive and undesirable side loading. Side loading can be applied to a digger derrick boom when, for example, a winch attached to the boom is used to lift a load that is positioned to a side of the boom, or when the truck is positioned on a grade such that the boom is operated at a lateral inclination. Side loading also often occurs when an auger-type digger coupled with the boom “corkscrews” into the ground due to the application of excessive pressure in driving the auger or when anchors are improperly installed. Excessive side loading can cause damage or destruction of the rotation drive mechanism, the boom turret, and the boom.
Systems have been developed for protecting mobile cranes, digger derricks and other rotating equipment against excessive side loads. U.S. Pat. No. 4,100,973 owned by ALTEC INDUSTRIES, for example, discloses a side load protection system that attempts to sense undesired side load levels and respond with appropriate action when pre-established limits have been exceeded. The system uses relief valves that open when pressure increases beyond a desired level in a hydraulic motor to allow the boom driven by the motor to slip or rotate toward the external load, thereby reducing the side load. Other side load protection systems shut down certain digger derrick operations once side load limits have been reached.
Although generally effective, existing side load protection systems are subject to limitations. For example, such systems are configured to allow a particular maximum side load to be applied to the boom system regardless of the position of the boom. Because most booms can safely withstand different side loads in different positions, such side load protection systems often operate inefficiently, allowing the boom to slip when subject to lower side loads than is necessary.